The pituitary gland is attached by a stalk to the region in the base of the brain known as the hypothalamus. In particular, follicle stimulating hormone (FSH) and luteinizing hormone (LH), sometimes referred to as gonadotropins or gonadotropic hormones, are released by the pituitary gland. These hormones, in combination, regulate the functioning of the gonads to produce testosterone in the testes and progesterone and estrogen in the ovaries, and also regulate the production and maturation of gametes.
The release of a hormone by the anterior lobe of the pituitary gland usually requires a prior release of another class of hormones produced by the hypothalamus. One of the hypothalamic hormones acts as a factor that triggers the release of the gonadotropic hormones, particularly LH, and this hormone is referred to herein as GnRH although it has also been referred to as LH-RH and as LRF. GnRH has been isolated and characterized as a decapeptide having the following structure: EQU p-Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH.sub.2
Peptides are compounds which contain two or more amino acids in which the carboxyl group of one acid is linked to the amino group of the other acid. The formula for GnRH, as represented above, is in accordance with conventional representation of peptides where the amino group appears to the left and the carboxyl group to the right. The position of the amino acid residue is identified by numbering the amino acid residues from left to right. In the case of GnRH, the hydroxyl portion of the carboxyl group of glycine has been replaced with an amino group (NH.sub.2). The abbreviations for the individual amino acid residues above are conventional and are based on the trivial name of the amino acid, e.g. p-Glu is pyroglutamic acid, His is histidine, Trp is tryptophan, Ser is serine, Tyr is tyrosine, Gly is glycine, Leu is Leucine, Arg is arginine, Pro is proline, Phe is phenylalanine and Ala is alanine. These amino acids together with valine, isoleucine, threonine, lysine, aspartic acid, asparagine, glutamine, cysteine, methionine, phenylalanine, and proline are generally considered to be the common, naturally occurring or protein-derived amino acids. Except for glycine, amino acids of the peptides of the invention are of the L-configuration unless noted otherwise.
It is well known that the substitution of D-amino acids for Gly in the 6-position of the GnRH decapeptide provides a peptide material having from about 1 to 35 times greater potency than does GnRH to effect the release of LH and other gonadotropins by the pituitary gland of mammalians. It is taught by K. U. Prasad et al. J. Med. Chem., Vol. 19, 492 (1976) that greater potency is also achieved by the substitution in the 3-position of 3-(1-naphthyl) Ala. The releasing effect is obtained when the GnRH analog is administered to a mammalian intravenously, subcutaneously, intramuscularly, orally, intranasally or intravaginally.
It is also known that substitution of various amino acids for His (or the deletion of His) at the 2-position of the GnRH decapeptide produces analogs having an inhibitory effect on the release of LH and other gonadotropins by the pituitary gland of mammalians.
There are reasons for desiring to prevent ovulation in female mammalians, and the administration of GnRH analogs that are antagonistic to the normal function of GnRH have been used to prevent ovulation. For this reason, analogs of GnRH which are antagonistic to GnRH are being investigated for their potential use as a contraceptive or for regulating conception periods. Such antagonists have also been found useful to regulate the secretion of gonadotropins in male mammals and can be employed as male contraceptives. It is desired to provide peptides which are strongly antagonistic to endogenous GnRH and which prevent secretion of LH and the release of steriods by the gonads of mammals.